Helicopter Vibrations

Vibrations are not always the good ones in Brian Wilson’s song — especially when they’re in helicopters. And it’s safe to say that helicopter pilots don’t get “excitations” when they occur. Although generally associated with the worlds of mechanics and maintenance, there are also some aerodynamically-related vibrations worthy of discussion in a training program.

Low frequency mechanical vibrations are associated with the main rotor blades, main rotor head or control linkages — and are usually slow enough that they can be counted. Troubleshooting airframe bumping and thumping from blades being out of track and balance with each other is almost an art form, and not all maintenance engineers are good at doing so. Every pilot has probably spent the greater part of a day “running up” the helicopter so that the engineer can smooth out the blades.

In the pioneer days of the Bell 47, engineers had little to work with to help solve blade tracking problems. It was not uncommon to see the intrepid engineer raising a broomstick handle with a blob of grease on the end ever so gently up to the blade tips at full RPM. As soon as light contact was made hopefully only the lower of the two blades had the grease smudge and then a trim tab adjustment would raise it to fly in the same plane as the other blade. More sophisticated tracking devices then appeared, such as the venerable Chadwick-Helmuth Vibrex kits, which were especially helpful with multiple blade systems where the grease trick had failings. These days, even more refined vibration analysis equipment and on-board monitoring systems are used to make your helicopter as smooth as a baby’s bottom, so to speak.

The Bell LongRanger, first introduced in 1974, had a revolutionary nodal beam pylon suspension system to support the main rotor transmission and the rotors above. The ride was so smooth and comfortable that blade tracking irregularities were almost imperceptible, and pilots needed a heightened awareness of even the smallest of vibrations to be able to report an irregularity to the engineers. LongRanger pilots back from the summer season who then flew a regular JetRanger immediately snagged the rotors for being out of track, when of course the problem was that they had been spoiled by flying the LongRanger.

Medium frequency vibrations are associated with the tail rotor, which rotates much faster than the main rotors. The tail rotor is usually easier to track and balance than the main rotor. With either rotor, if the source of a presumed rotor vibration is unrelated to the blades, this must be identified and rectified first — or you could be tracking the blades ad infinitum without making any progress. Good track and balance technicians are worth their weight in gold.

High frequency vibrations are associated with the engine bearings, cooling blower fan blades, the tail rotor driveshaft, or any other component spinning at high revolutionary speeds. Main rotors rotate at close to 360 revolutions per minutes (r.p.m.), with tail rotors generally spinning six times faster than that. Piston crankshafts normally rotate at 3,000 r.p.m., so if the tail is spinning at 2,160 r.p.m., it takes a discernible “hear and feel” to isolate the source of the vibrations.

Ground resonance is a vibration problem associated with oleo landing-geared multi-bladed helicopters. Ground resonance is usually caused by an imperfect landing which jars the rotor system into an oscillating imbalance — and could cause the eventual destruction of the helicopter. If recognized in time, this hazard requires immediate shutdown — or better yet, an immediate takeoff back to the hover to eliminate it. Many people watch these catastrophic events on YouTube with morbid curiosity. Human nature at its best.

Pilots trying to fly too quickly should heed the airframe vibration warning of the onset of retreating blade tip stall, blade compressibility, and the aerodynamic imbalance of reverse flow. The remedy, as you can imagine, is simply to slow down.

Some helicopter vibrations are aerodynamically related. Good utility helicopter pilots work with the friendly vibration at translation all the time. Coincidental with the point of translation, sometimes referred to as effective translational lift (ETL), is a pronounced vibration caused by the transverse flow effect when the horizontal airflow over the front rotors begins to descend through the rear half of the disk. Utility pilots like to work heavy loads just ahead of this vibration, because they can control the weight of the helicopter with high power just ahead of translation. Pilots who fly helicopters exactly as they do airplanes have no understanding of this, because they are always flying at speeds well above translation, presumably for safety reasons.

Utility pilots are also keenly aware of a potentially threatening vibration with a very different ominous feel, associated with the onset of the dangerous vortex descent at low speeds just behind translation. Pilots will keep nudging forward on approach just enough to stay ahead of both the vortex descent and translation. It’s much like surfing; standing just forward enough on the board to keep propelling forward, or you will fall back off the wave into the swell.

Helicopters are exhilarating to fly, but keeping all the moving parts running smoothly and vibration-free is an ongoing task for both pilots and engineers. Using the vibration at translation to advantage and working hard to source the origin of unwelcome mechanical vibrations are important aspects of helicopter flight that you need to come to grips with in a good helicopter training program.